Ok, time for another round of picture updates. The tank has (hopefully!) been waterproofed.
Because the tank has minimal bracing I decided to use fiberglass to reinforce the structure of the submerged section. I'd never worked with fiberglass before so, as with everything else about this build, it was a learning experience and I got better as a I went along. Here's how I went about it once I had the whole process figured out.
First I tilted the tank at an angle so that all residual epoxy resin would pool into the seams, deeply penetrating them and effectively creating a fillet. This was a little cumbersome because I had to reposition the tank for every seam, but it worked out very well.
I coated the seams with an initial layer of epoxy to saturate the wood and provide an initial barrier coating. I also dripped a little extra epoxy into the seams to make a slightly thicker fillet.
After this first layer had dried and was no longer tacky, but not completely cured, I layed out a strip of fiberglass cloth into the corner. I just used the cheap, lightweight Elmer's brand cloth from Lowes since I figured it would be adequate for my purposes. The lighweight cloth is pretty easy to work with. I found it made things easier if I took my time to make sure it was cut straight before starting.
Here's the strip wetted out with epoxy. After brushing it on I used the flat end of a stir stick and a gloved finger to really push it into the seams and force out any air bubbles. Make sure to work out the air bubbles while it's wet and you still can. Then I dripped a little extra epoxy on to really get a nice thick layer in the seams.
After waiting a few hours for the epoxy to gel (but not harden) I used my trusty paring knife to trim off the excess cloth to get a nice clean edges. I found that you shouldn't try to trim the cloth before it sets up or you'll pull it out of place and introduce air bubbles. Similarly, if you wait until it's completely cured it becomes too hard and sharp, making it difficult and potentially dangerous to cut. Leave an adequate strip of dry cloth to grip on to and do it when it's tacky and rubbery.
And there you have the finished reinforced seam. Hopefully it's completely sealed and will resist the formation of stress fractures.